1. Field of the Invention
The present invention relates to a multiple frequency-adaptable surface-mount type antenna designed for use in mobile communication equipment such as a cellular mobile phone, for allowing signal transmission and reception at two different frequency bands, and also relates to an antenna apparatus and a wireless communication apparatus that employ the surface-mount type antenna.
2. Description of the Related Art
In recent years, wireless communication apparatuses designed for multiple frequency bands, such as cellular mobile phones, have been coming into wider and wider use that are usable, only with a single wireless communication unit, in a plurality of applications including GSM (Global System for Mobile Communications), DCS (Digital Cellular System), PDC (Personal Digital Cellular), PHS (Personal Handyphone System), GPS (Global Positioning System), and Bluetooth System. Moreover, in consideration of carryability, down-sizing has come to be increasingly demanded of a communication terminal for constituting such an apparatus.
Such a widely-used wireless communication apparatus has succeeded in reducing the size by utilizing appropriate surface-mount type antennas of various design.
Now, a description will be given below as to examples of a related art surface-mount type antenna designed for two different frequency bands (hereinafter referred to simply as “2-frequency surface-mount type antenna”) and an antenna apparatus incorporating the same, with reference to perspective views shown in FIGS. 12 and 13 and a developed view shown in FIG. 14 (refer to Japanese Unexamined Patent Publication JP-A 2001-298313).
In the 2-frequency surface-mount type antenna 61 shown in FIG. 12, reference numeral 62 denotes a base body having a rectangular-parallelepiped shape, 63 and 64 denote a feeding electrode, and 65 and 66 denote a radiating electrode.
The related 2-frequency surface-mount type antenna 61 becomes able to provide a 2-frequency operation function; that is, becomes able to operate at two different frequencies, by making a change to the lengths of the radiating electrodes 65 and 66. For example, of the two different frequencies, a lower frequency f1 is obtained by increasing the length of the radiating electrode 65, whereas a higher frequency f2 is obtained by decreasing the length of the radiating electrode 66.
In FIG. 13, reference numeral 71 denotes a surface-mount type antenna, which is mounted on a mounting substrate 78 to constitute an antenna apparatus 80. In the surface-mount type antenna 71 shown in FIG. 13, reference numeral 75 denotes a base body having a rectangular-parallelepiped shape; 74 denotes a feeding electrode; and 72 and 73 denote radiating electrodes. In addition, in the mounting substrate 78, reference numeral 77 denotes a feeding terminal, and 76 denotes a ground conductor layer.
The related art surface-mount type antenna 71 becomes able to provide a 2-frequency operation function; that is, becomes able to operate at two different frequencies, by making a change to pitches of the radiating electrodes 72 and 73. On a side of the base body 71, a pitch of the spiral radiating electrode 73 connected to the feeding electrode 74 is made coarse, and a pitch of the spiral radiating electrode 72 connected to the radiating electrode 73 is made dense.
Such a surface-mount type antenna 71 is mounted on a surface of the mounting substrate 78 by connecting the feeding electrode 74 to the feeding terminal 77, whereby 2-frequency antenna apparatus 80 is constituted.
Moreover, in FIG. 14, reference numeral 81 denotes a surface-mount type antenna. In the surface-mount type antenna 81, reference numeral 84 denotes a feeding electrode, 82 and 89 denote a feeding-side radiating electrode, 83 and 87 denote a non-feeding-side radiating electrode, and 88 denotes a ground electrode.
By virtue of adaptability of the feeding-side radiating electrodes 82 and 89 to higher-order mode frequencies and arrangement of the non-feeding-side radiating electrodes 83 and 87 and a branch electrode as well, the related surface-mount type antenna 81 becomes able to provide a multi-frequency operation function; that is, becomes able to operate at a plurality of different frequencies. It is thus necessary to provide the radiating electrodes 83 and 87, as a current flowing path of different electrical length, other than the feeding-side radiating electrodes 82 and 89.
In addition, as a multiple frequency-adaptable antenna, for example, there is disclosed an antenna for a mobile communication terminal that is adapted to be used in plural frequency bands including frequency bands different from a predetermined frequency band by connecting a grounded capacity of an antenna element to the antenna element for the predetermined frequency band to change a value of the predetermined frequency band (refer to Japanese Unexamined Patent Publication JP-A 2002-232232). According to this disclosure, since it is necessary to insert a switch in series in a transmission path for switching transmission and reception signals, a problem of a signal transmission loss is caused.
In addition, the related 2-frequency surface-mount type antenna 61 as shown in FIG. 12 poses the following problems. In this construction, the feeding electrodes are two in number and are disposed independently of each other, and so are the radiating electrodes. This configuration, although it is advantageous in easiness of frequency adjustment and matching control, makes miniaturization difficult.
Furthermore, even if the dielectric constant of the base body 62 is increased, and the radiating electrode is shortened by exploiting a short wavelength effect in an attempt to make the antenna compact, due to the occurrence of intense mutual electromagnetic-field interference between the radiating electrodes 65 and 66, the antenna characteristics are deteriorated.
In addition, the related surface-mount type antenna 71 shown in FIG. 13 poses the following problems. In this construction, in order to match an operation frequency of the surface-mount type antenna 71 to a lower frequency f1 and a higher frequency f2 of a radio signal used in a communication system, it is necessary to adjust lengths and pitches (spacings) of the spiral radiating electrodes 72 and 73, and the adjustment requires many labor hours.
Furthermore, in the case of using only one feeding electrode, there arises significant mutual interference between signals of two different frequencies. Eventually, the signals turn into sources of noise with respect to each other.
In addition, there also arises the following problem. When it is attempted to increase a dielectric constant of the base body 75 to reduce a size of the surface-mount type antenna 71, since an unnecessary resonance mode occurs unexpectedly between the spiral long radiating electrodes 72 and 73 and the ground conductor layer 76, and stable antenna characteristics adaptable to two frequencies is not obtained, it is difficult to reduce a size of the surface-mount type antenna 71.
Further, in the antenna element disclosed in JP-A 2002-204120, there arises a problem that it is difficult to apply surface mounting to a mounting substrate.
The surface-mount type antenna 81 disclosed in JP-A 2001-298313 as shown in FIG. 14 also poses the following problems. In order to achieve multiple-frequency antenna operation, in the surface-mount type antenna 81, the non-feeding-side radiating electrodes 83 and 87 are provided as a current flowing path of different electrical length by means of electrode branching technique. By virtue of the non-feeding-side radiating electrodes 83 and 87, the multiple-frequency antenna operation can be achieved. However, in general, a non-feeding side electrode is placed in the proximity of a feeding side electrode, and thus it may be able to function as an antenna on its own by exploiting an electromagnetic field generated at the feeding side electrode. It is therefore inevitable that mutual electromagnetic-field interference takes place between the feeding side electrodes and non-feeding side electrodes. Moreover, the non-feeding-side radiating electrodes 83 and 87 are so formed as to exhibit a branched structure with respect to a single ground electrode 88 (non-feeding electrode, as exemplified). This makes it difficult to strike a proper balance of matching between the non-feeding-side radiating electrodes 83 and 87. Moreover, the non-feeding-side radiating electrodes 83 and 87 are so formed as to exhibit a branched structure with respect to a single ground electrode 88 (non-feeding electrode, as exemplified). This makes it difficult to strike a proper balance of matching between the non-feeding-side radiating electrodes 83 and 87. That is, since in the surface-mount type antenna 81, the radiating electrodes 83, 87 are formed to exhibit a branched structure in order to make it possible to respond to plural frequencies with a single feeding terminal, it is difficult to achieve impedance matching in each of the blanched radiating electrodes.
Furthermore, the feeding-side radiating electrodes 82 and 89 are adaptable to two frequencies: a fundamental frequency and a higher-order mode frequency. In order to make frequency adjustment on an individual basis, the electrode is designed to vary in breadth from part to part. In the surface-mount type antenna 81, frequency adjustment is made with respect to the fundamental frequency and the higher-order mode frequency by narrowing a certain part of the electrode pattern. However, further additional frequency adjustment cannot be achieved without changing the electrode pattern. This makes frequency adjustment extremely difficult. With regard to this it may be conceivable that frequency adjustment can be made by widening the electrode pattern in part. In the surface-mount type antenna 81, however, if its electrode pattern is partially widened, at the time of trimming in the widened portion of the pattern, there will be a change in the current flowing path inconveniently. This makes delicate frequency adjustment difficult.